One common chip-type electronic part is known as a chip inductor, a chip resistor, a chip-type multilayer ceramic condenser (capacitor), a chip-type multilayer ceramic capacitor, or a chip thermistor. Such a chip-type electronic part includes a chip-type element, essentially including a ceramic sintered body, an inner electrode provided inside of the ceramic sintered body, and an outer electrode provided on both end surfaces of the chip-type element so as to be electrically connected with the inner electrode. These components are mounted on a substrate by soldering the outer electrode.
The above chip-type electronic part is connected with an electric circuit on a substrate by an outer electrode. The quality or performance thereof significantly affects products' electric properties, reliability, and mechanical properties.
In conventional chip-type electronic parts, an outer electrode is formed by mixing an noble powdered metal such as Ag, Pd, and Pt, and an inorganic bonding material, kneading the mixture in an inorganic vehicle, coating both end surfaces of a chip-type element with the resulting conductive paste, and firing the coated body at approximately 500 to 800° C. (referred to as “sintered electrode”).
In fact, a conventional outer electrode consisting only of a sintered outer electrode has the following two technical problems. First, such an outer electrode allows for the formation of a nickel-plated film and a tin or tin/palladium plated electrode layer. The nickel-plated film is configured to prevent solder leaching, i.e., dissolution of an outer electrode on the contact surface with the solder. On the other hand, the tin or tin/palladium plated electrode layer is designed to restrict decline in soldering performance by oxidation of the nickel-plated film. Firing conditions of forming an outer electrode determine electric properties of a chip-type electronic part to be obtained after formation of the plated film. This characteristic unfortunately fails to produce highly reliable chip-type electronic parts. Secondly, the structure of an outer electrode formed of sintered metal with high hardness can cause cracking in a ceramic sintered body as part of a chip-type element in temperature cycles in operation.
To solve the aforementioned problems, it is proposed that in such a chip-type electronic part, an end surface of the chip-type element is coated with a resin composition obtained by dispersing a conductive filler such as, particularly, a silver powder in a binder resin such as, particularly, an epoxy resin to be cured. Accordingly, a resin electrode layer as a conductive layer is formed as part of an external terminal electrode. The resin electrode layer is used to relax thermal expansion of the external terminal electrode when thermal stress is applied on the external terminal electrode and prevent generation of cracking (e.g., Patent Documents 1 to 3).
Another chip-type electronic part is disclosed in e.g., Patent Document 4 as a multilayer ceramic capacitor using a conductive resin composition containing a silicone resin consisting of polydimethylsiloxane (PDMS) and a conductive metal particle (conductive filler). This type of multilayer ceramic capacitor includes a conductive resin layer consisting of the above conductive composition as a conductive layer between an outer electrode formed on an end surface of a ceramic element and a plated layer of the outermost layer. The conductive metal particle is composed of copper, silver, or copper coated with silver on the surface. The multilayer ceramic capacitor is excellent in moisture resistance in the conductive resin layer, and the bending strength of the outer electrode in the multilayer ceramic capacitor can be improved. The conductive resin layer is a silicone resin including a binder component composed of PDMS, and high in thermal expansion relaxation when thermal stress is applied on the external terminal electrode on an end surface of a ceramic element.
Meanwhile, an alternative material of a conductive filler consisting of the above metal powder or a metal particle is disclosed in e.g., Patent Document 5 as a metal-coated resin particle plated with conductive metal on the surface of a spherical resin particle. In Patent Document 5, the resin core particle may be an acrylic resin or a styrene resin. The metal-coated resin particle is flexible and of low-specific gravity, with a core consisting of a resin, in place of a conductive metal powder consisting of a conventional silver powder.